Burn Rate Sensitization of Solid Propellants Using a Nano-Titania Additive

ABSTRACT

Adding nanoparticles as a catalyst to solid propellant fuel to increase and enhance burn rates of the fuel by up to 10 times or more and/or modifying the pressure index. A preferred embodiment uses TiO 2  nanoparticles mixed with a solid propellant fuel, where the nanoparticles are approximately 2% or less of total propellant mixture. The high surface to volume ratio of the nanoparticles improve the performance of the solid propellant fuel.

This invention claims the priority of U.S. Provisional PatentApplication No, 60/705,395 filed on Aug. 4, 2005.

FIELD OF THE INVENTION

This invention relates to nanoparticles, in particular to methods ofmaking and using nanoparticle additives such as TiO₂ as catalysts toenhance solid propellant burn rates where the high surface-to-volume ofthe nanoparticles provides greater benefit over traditional additives.

BACKGROUND AND PRIOR ART

Additives comprising fractions of a percent to several percent of solidpropellant mixtures have been considered through the years and arecommonly employed in many rocket propellants and explosives. Variousadditives include burn-rate modifiers (e.g., ferric oxide, metal oxides,and organometallics); curing agents; and plasticizers. In certain cases,additions of small (<5% by weight) amounts of powdered material to thepropellant mixture have been shown to increase or otherwise favorablymodify the burn rate as described in T B Brill, B T Budenz 2000 “FlashPyrolysis of Ammonium Percholrate-Hydroxyl-Terminated-PolybutadieneMixtures Including Selected Additives,” Solid Propellant Chemistry,Combustion, and Motor Interior Ballistics, Vol. 185, Progress inAstronautics and Aeronautics, V Yang, T Brill, W-Z Ren (Ed.), AIAA,Reston, Va.: 3-32. For example, it has been observed by a fewinvestigators that TiO2 (titania) particles may enhance stability bycreating burn rates that are insensitive to pressure over certainpressure ranges as disclosed in U.S. Pat. No. 5,579,634 issued to Tayloron Dec. 3, 1996. It is suspected that other organometallic particles mayproduce these and other favorable traits described in Brill.Nanoparticle additives may have an even further influence on the burnrate because of their high surface-to-volume ratios.

Over the past few years, nanoparticles of many different compounds andcombinations have received considerable attention in the scientific andengineering research communities. This surge of activity is a result ofthe many favorable characteristics certain materials and applicationsexhibit when nanoparticles are involved in some fashion. Benefits arecertainly seen in composite Al/AP/HTPB-based solid propellantformulations when the micron-scale metal fuel (i.e., Al) is replaced bynanoscale particles as described in P Lessard, F Beaupre, P Brousseau,2001 “Burn Rate Studies of Composite Propellants Containing Ultra-FineMetals,” Energetic Materials—Ignition, Combustion and Detonation,Karlsruhe, Germany; 3-6 Jul. 2002: 88. pp. 1-13 and in A Dokhan, E WPrice, 3M Seitzman, RK Sigman, “Combustion Mechanisms of Bimodal andUltra-Fine Aluminum in AP Solid Propellant,” AIAA Paper 2002-4173, July2002. However, little research has been done on the effect of nanosizedadditives such as organometallics and related burn rate-enhancing andsmoke-reducing compounds.

Other prior art made of record includes U.S. Pat. No. 6,503,350 issuedto Martin on Jan. 7, 2003, describes propellants such as may be used insolid rocket motors. In one preferred embodiment, the propellantcomprises one high energy propellant composition comprising ahomogeneous mixture of fuel and oxidizer having a predeterminedfuel/oxidizer ratio, wherein individual fuel particles are generallyuniformly distributed throughout a matrix of oxidizer, and a low energypropellant comprising a fuel and oxidizer. The amounts of the twopropellants are present in amounts which achieve a preselected burnrate.

U.S. Pat. No. 6,605,167 issued to Blomquist on Aug. 12, 2003, disclosesan autoignition material that includes a plurality of agglomerates. Eachagglomerate comprises an oxidizer material particle. A plurality ofmetal fuel particles are disposed on the oxidizer material particle. Themetal fuel particles are present in a weight ratio effective tochemically balance the oxidizer material particle. The metal fuelparticles exothermically react with the oxidizer material particle whenthe autoignition material is exposed to a temperature of about80.degree. C. to about 250.degree. C. A thin binder film adheres themetal fuel particles to the oxidizer material particle and maintains themetal fuel particles in intimate contact with the oxidizer particles.

U.S. Pat. No. 6,270,836 issued to Holman on Aug. 7, 2001, describessol-gel preparation of particles. The gel-coated microcapsules haveimproved mechanical stress- and flame-resistance. A method for makingthe gel coated microcapsules is also provided. Phase change materialscan be included in the microcapsules to provide thermal control in awide variety of environments.

U.S. Pat. No. 6,086,692 issued to Hawkins, et al. on Jul. 11, 2000,describes an advanced design for high pressure, high performance solidpropellant rocket motors and describes a solid rocket propellantformulation with a burn rate slope of less than about 0.15 ips/psi overa substantial portion of a pressure range and a temperature sensitivityof less than about 0.15%/.degree F. A high performance solid propellantrocket motor including the solid rocket propellant formulation is alsoprovided. The solid rocket propellant formulation can be cast in a grainpattern such that an all-boost thrust profile is achieved.

U.S. Pat. No. 4,881,994 issued to Rudy, et al. Nov. 21, 1989, disclosesferric oxide as burn rate catalyst and use of isocyanate curing agent.The patent describes a method of making a ferric oxide burning ratecatalyst that results in a highly active, finely divided burning rateenhancing catalyst. The ferric oxide burning rate catalyst isparticularly adapted for use in a composite solid rocket propellant.This process provides an ultra pure, highly active, finely dividedburning rate catalyst.

U.S. Pat. No. 4,658,578 issued to Shaw, et al. on Apr. 21, 1987,discloses improved igniter compositions for rocket motors are providedwhich, when cured, are non-volatile and are capable of igniting undervacuum conditions and burning steadily at reduced pressures.

U.S. Pat. No. 4,655,858 issued to Sayles on Apr. 7, 1987, describesmetal/oxidant agglomerates for enhancement of propellant burning ate areprepared from a finely divided metal (aluminum, boron, titanium, etc.),ammonium perchlorate, and a small quantity of the same binder materialthat goes into the manufacture of the propellant, such as,hydroxyl-terminated polybutadiene crosslinked with a polyisocyanate.

SUMMARY OF THE INVENTION

A primary objective of the invention is to provide methods, systems,apparatus and devices to provide a titania nanoparticle additive forcomposite solid propellants.

A secondary objective of the invention is to provide methods, systems,apparatus and devices to provide a titania nanoparticle additive forcomposite solid propellants for improved performance due to their highsurface to volume ratio.

A third objective of the invention is to provide new methods, systems,apparatus and devices for the addition of titania nanoparticle additivesat about 0.4% of the total propellant mass to produce an impact on theburn rate of solid propellants up to ten times or more at variouspressures.

A first preferred embodiment of the invention provides a composite solidpropellant having a catalyst. Nanoparticles of TiO₂ additive are mixedwith solid propellant fuel to produce a final propellant mixture,wherein the nanoparticles of TiO₂ act as the catalyst to modify the burnrate of the composite solid propellant. The TiO₂ additive is less thanapproximately 2.0% of the composite solid propellant by mass.

For the second embodiment, the novel method for enhancing solidpropellant burn rates that includes the steps of providing a solidpropellant fuel and nanoparticles of TiO₂ additive as a catalyst, andmixing the nanoparticles of TiO₂ additive with the solid propellant fuelto modify burn rate of the fuel. The nanoparticles of TiO₂ additive areproduced by first mixing Isopropanol anhydrous and 2,4-Pentanedionetogether, mixing titanium Isoproproxide with the solution, then mixingDI water for hydrolysis to produce a final mixture. The final mixture isaged to produce the nanoparticles of TiO₂ additive.

Further objects and advantages of this invention will be apparent fromthe following detailed description of preferred embodiments which areillustrated schematically in the accompanying drawings.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a graph showing the deconvoluted Ti(2p) peaks obtained fromnano-T_(i)O₂ power synthesized using the sol-gel technique.

FIG. 2 is a graph showing the burn rate results for nanoparticle titaniaadditive and baseline mixture with no titania.

FIG. 3 is a flow diagram of the procedure for producing nanoparticles.

FIG. 4 is a flow diagram of the procedure for enhancing solid propellantburn rates using the nanoparticles of TiO2 produced using the procedureshown in FIG. 3.

FIG. 5 is a flow diagram of a method of producing a composite solidpropellant.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Before explaining the disclosed embodiments of the present invention indetail it is to be understood that the invention is not limited in itsapplication to the details of the particular arrangements shown sincethe invention is capable of other embodiments. Also, the terminologyused herein is for the purpose of description and not of limitation.

The apparatus, methods, systems and devices of the present inventionencompassed adding nanoparticles of TiO₂ as a catalyst to solidpropellant fuel such as R-45 Binder, MDI Cure Agent, monomodial Ammoniumperchlorate (Fe3O₂). A preferred mixture has nanoparticles beingapproximately 0.4% of total propellant mass of the mixture, where thatcatalyst can increase and enhance burn rates of the fuel up to ten timesor more. The high surface-to-volume ratio of the nanoparticles have animportant impact on the performance of the solid propellant ftiel.

Materials for the synthesis of the TiO₂ particles included Isopropanolanhydrous, 2,4-Pentanedione, and Titanium isopropoxide purchased fromSigma Aldrich. Deionized (DI) water was also used. The procedure for theTiO₂ particles involved a sol-gel technique. As shown in FIG. 3, thetechnique is based on the hydrolysis of liquid precursors and theformation of colloidal sols. Specifically, 100 ml of Isopropanolanhydrous and 2 ml of 2,4-Pentanedione were added together and stirredfor 20 minutes. Titanium Isopropoxide was then added to the solution andstirred for 2 hours. DI water was then added for hydrolysis and stirredfor an additional 2 hours, and the solution was left to age for 12hours. This procedure produced a yield of 1.6 g of nanoparticles.

X-Ray Photoelectron Spectroscopy (XPS) was used to verify the chemicalstructure of the TiO2 particles. The resulting XPS data confirm theformation of TiO₂ particles due to the 2p3 peak at 458.89 eV of bindingenergy (FIG. 1) according to typical peak formation for TiO₂.Transmission Electron Microscopy (TEM) (Philips Technai transmissionelectron microscope) was also used to study the size and distribution ofthe particles. The TEM results revealed nano-sized arrays of particleswith diameters on the order of 10 nm with a narrow size distribution.

As shown in the flow diagram of FIG. 4, the procedure for enhancingsolid propellant burn rates involves providing a solid propellant fueland mixing in the nanoparticles of TiO2 produced using the procedureshown in FIG. 3. For the final propellant mixture, the amounts for eachcomponent consisted of the following by mass: the fuel (3-μm Al+titaniaadditive) was 20%, the oxidizer (200-μm monomodal Ammonium Perchlorate,AP) was 67.5%, Fe3O2 was 0.5%, the R-45M binder (HTPB) was 10.5%, andthe cure agent (MDI) was 1.5%. The TiO₂ additive was 2.0% of the fuel bymass, or 0.4% of the entire mixture.

FIG. 5 is a flow diagram of a method of producing a composite solidpropellant. The mixing procedure began by mixing all components into themixer, starting with the HTPB followed by the Fe₃O₂, the aluminumpowder, and the titania solution. The mixture was mixed for 20 minutesunder a vacuum and then left under the vacuum until the solvent wascompletely evaporated (2 days). Heating tape was applied to the mixtureto heat the mixing vessel to 50° C. to help evaporate the Isopropanolsolvent from the titania solution. After all of the Isopropanol wasevaporated, the AP was added, and the mixture was mixed under vacuum for25 minutes. The MDI curing agent was added, and mixing continued for 5minutes. The mixture was put under Nitrogen pressure at about 10 atm tocompact the propellant for extruding. Teflon tubing with a 64-mm outerdiameter was used to extrude the propellant samples from the mixture forburn testing. Several strands were extruded and left to cure for 2 daysat room temperature. A high-pressure strand burner was used to measurethe burn rate of the propellant samples.

Burn rates were determined from two different measurements: pressure andlight emission. Both diagnostics provide information leading to thetotal burn time. The burn rates (cm/s) were calculated by dividing themeasured length of each sample by the total burn time. Further detailson the propellant mixing and burning apparatus and procedures aredescribed in R. Carro et al., “High-Pressure Testing of Composite SolidPropellant Mixtures: Burner Facility Characterization,” AIAA Paper, 41stAIAA/ASME/ASEE Joint Propulsion Conference & Exhibit (2005).

The propellant samples were burned in the strand burner at pressuresranging from 43 to 250 atm. FIG. 2 presents the burn rate results of thepresent mixture containing the nano-Titania compared to the results of abaseline mixture from a separate study described in J Arvanetes et al,“Burn Rate Measurements of AP-Based Composite Propellants at ElevatedPressures,” 4^(th) Joint Meeting of the U.S. Sections of the CombustionInstitute (2005) containing no additive. In other words, the entire fuelcomposition was Al. The mixture with the TiO₂ nanoparticles shows asignificant increase in the burn rate as a function of pressure—almost afactor of ten over a range of pressures. This increase in the burn ratemay come from the fact that titania nanoparticle additives greatlyincrease the surface area to volume ratio of the titania additive. Thetitania nanoparticles acted as catalysts to the burning rate.

The results confirm that the addition of titania nanoparticle additivesat about 0.4% of the total propellant mass has a definite impact on theburn rate of solid propellants at various pressures. Future studies arerequired to further verify these results, including repetition oftitania nanoparticle burns with larger pressure ranges, experimentationon the percentage of additive used in the propellant, consideration ofother organometallic nanoparticle additives, conduction of newsuspension methods of additives in various solvents, and exploration ofstructural characteristics and physical properties of the final product.

The application of nanotitania to solid propellants is not limited toAP/HTPB/Aluminum mixtures only, but can be applied to non-metallizedcomposite propellant mixtures (i.e., no aluminum) of AP/HTPB. Otheroxidizers and binders in place of AP (ammonium perchlorate) and HTPB(Hydroxyl-terminated Polybutadiene) can be used as the baselinecomposite propellant.

While the invention has been described, disclosed, illustrated and shownin various terms of certain embodiments or modifications which it haspresumed in practice, the scope of the invention is not intended to be,nor should it be deemed to be, limited thereby and such othermodifications or embodiments as may be suggested by the teachings hereinare particularly reserved especially as they fall within the breadth andscope of the claims here appended.

1-4. (canceled)
 5. A method for enhancing solid propellant burn rates,comprising the steps of providing a solid propellant fuel; providingnanoparticles of TiO₂ additive; and mixing the nanoparticles of TiO₂additive with the solid propellant fuel, wherein the nanoparticleadditive function as a catalyst to modify burn rate of the fuel.
 6. Themethod of claim 5, wherein the providing nanoparticles of TiO₂ additivecomprises the steps of: first mixing Isopropanol anhydrous and2,4-Pentanedione together to produce a first mixed solution; secondmixing titanium Isoproproxide with the first mixed solution to produce asecond mixed solution; third mixing DI water with the second mixedsolution for hydrolysis to produce a final mixture; and aging the finalmixture to produce the nanoparticles of TiO₂ additive.
 7. The method ofclaim 6, wherein the first mixing step comprises the step of: addingapproximately 100 ml of Isopropanol anhydrous to approximately 2 ml of2,4-Pentanedione; and stirring for approximately 20 minutes.
 8. Themethod of claim 5, wherein the mixing step includes the step of:uniformly disbursing the nanoparticles of TiO2 within the solidpropellant fuel.
 9. The method of claim 5, further comprising the stepof: curing the mixture by heating the mixture.
 10. The method of claim5, further comprising the step of: increasing burn rate of the solidpropellant fuel by approximately 2 to approximately 10 times over thebaseline propellant without the additive.
 11. The method of claim 5,further comprising the step of: tailoring the burning rate of the solidpropellant fuel by changing the pressure index (i.e., sensitivity ofburning rate to pressure) over the baseline propellant without theadditive.
 12. The method of claim 5, wherein the mixing step comprisesthe step of: adding nanoparticles of TiO₂ to the solid propellant fuelas approximately 2.0% of the total propellant by mass.
 13. The method ofclaim 5, wherein the mixing step comprises the step of: addingnanoparticles of TiO₂ to the solid propellant fuel as approximately 0.4%of the mixture.
 14. The method of claim 5, further comprising the stepof: selecting a surface to volume ratio of the nanoparticles of TiO₂ toenhance catalytic properties.
 15. (canceled)